Hollow components such as airfoils (fan blades, compressor blades, and turbine blades) for gas turbine engines are used by engine manufacturers to reduce weight and to increase the operating engine efficiency. As engine manufacturers design engines that produce increased thrust, larger airfoils are needed. As engine size increases, reducing engine component weight becomes increasingly important. High bypass engines require very large diameter fans, and the larger fans make reducing the airfoil, or fan blade, weight while maintaining structural integrity critical. Hollow titanium fan blades help to decrease fan blade weight and provide a blade that can endure applied dynamic loads during engine operation.
Titanium hollow fan blades are fabricated using diffusion bonding often in conjunction with superplastic forming processes. Diffusion bonding forms a metallurgical bond between parts by pressing the parts together at elevated temperatures and pressures. Bonding of the parts occurs by the diffusion of atoms between adjacent part faces. Diffusion bonding provides essentially parent metal joint strength. Localized flow at the mating faces is essential to get full contact and to permit diffusion.
Superplastic forming relies on the property of certain metals, including titanium alloys, to exhibit high tensile or compressive material deformation with a minimal tendency toward local necking of the part, when the metal is exposed to time, temperature, and strain conditions within certain ranges. Both diffusion bonding and superplastic forming allow a variety of lightweight, high strength parts to be manufactured and are ideal for forming hollow titanium airfoils for gas turbine engines.
Modern hollow airfoils, such as blades and vanes, have a convex wall (the suction wall), a concave wall (the pressure wall), and a series of internal support ribs or protrusions extending spanwise (radially) and/or chordwise between the concave and convex walls, that define at least one internal cavity to reduce the weight of the airfoils while maintaining structural requirements. The walls present a problem during the diffusion bonding process. The ribs are matched together at a bond line, and the surface area of the ribs (as a function of bond plane area) is usually less at the center of the airfoil than the surface area of the edges of the airfoil, at the bond plane. The loading required for diffusion bonding causes material to flow in the direction of least resistance, which is primarily in the net direction of the applied forces, or normal to the direction of applied forces if the forces are equal and opposite. The amount of material flow is partly a function of the amount of material at the bond line. During diffusion bonding, the airfoil walls are placed in compression near the airfoil edges, as a result of material flow, and tend to buckle. Buckling of the walls distorts the structural characteristics of the part and interferes with the airflow across the airfoil. Past solutions to prevent buckling have included increasing wall thickness to resist the compressive forces or shortening the span of the cavities. However, both of these solutions increase the weight of the airfoil.
A hollow airfoil design that can be diffusion bonded without buckling and without increasing the weight of the airfoil is needed.